1. Field of the Invention
The present invention relates to a cooling structure for a piston in an internal combustion engine. The invention also relates to an internal combustion engine provided with the piston having such cooling structure.
2. Description of the Related Art
A piston for an internal combustion engine (hereinafter sometimes simply referred to as “piston”) reciprocates within a cylinder bore when combustion occurs in the engine. Thus, the piston is required to not only be rigid enough to withstand high-speed motions and heat deformation, but also be light and have lubrication and cooling performance. For instance, the related art described in the Japanese Utility Model application publication No. JP-U-7-17937 offers the piston having a reduced weight, and improved lubrication and cooling performance.
The piston described in the Japanese Utility Model application publication No. JP-U-7-17937 has recesses formed in the forward (Fr) and rearward (Rr) portions (toward the front and the rear of the engine, respectively) of the piston to reduce the weight of the piston. Each weight-reducing recess is formed by molding in the outer upper section of a pin boss. The pin boss has an insertion hole for a piton pin. The piston has additional recesses in the pin boss, which are opened to the respective weight-reducing recesses. These weight-reducing recesses in the piston and recesses in the pin boss are provided to reduce the weight of the piston.
Oil is sprayed from an oil jet and delivered to the weight-reducing recesses and the pin boss recesses to cool the piston. Also, each pin boss recess is communicated with the piston pin hole in the pin boss to direct the oil delivered to the pin boss recess to the piston pin hole for lubricating the piston pin.
As described above, the piston has the weight-reducing recesses in its Fr and Rr portions. However, in an attempt to cool this piston using the oil delivered from the oil jet to the undersurface of the piston head, the weight-reducing recesses prevent heat from flowing. This results in a drawback of insufficient cooling of areas surrounding the weight-reducing recesses, and therefore, an increase in temperature in such areas. More specifically, the areas surrounding the weight-reducing recesses include the Fr and Rr portions of a land into which a piston ring is fitted, and the Fr and Rr circumferential portions of the piston head or the uppermost section of the piston.
The Japanese Utility Model application publication No. JP-U-7-17937 also describes that the oil is sprayed from the oil jet and splashed directly onto the weight-reducing recesses in the piston, thereby effectively cooling the area surrounding the weight-reducing recesses, and therefore cooling the entire piston. In this case, however, an individual oil jet is required to cool each of the areas surrounding the weight-reducing recesses formed respectively in the Fr and Rr portions of the piston. Therefore, two oil jets are required per piston. This increases the number of the components used in the piston, and thus increases the load on the oil pump undesirably.